A system for distributing pay television signals is described in a patent application entitled "Protected Television Signal Distribution System", Ser. No. 215,044, filed Dec. 10, 1980, assigned to the assignee of the present invention.
Briefly, in the above cited patent application, television signals are scrambled by suppressing the amplitude of horizontal and vertical synchronizing pulses so that the signal is not viewable on an ordinary television receiver. The timing information necessary to restore the horizontal and vertical synchronizing pulses is provided as amplitude modulation (AM) on the audio carrier.
Specifically, an AM pulse appears on the audio carrier in timed coincidence with the horizontal sync pulse. The vertical synchronizing interval is indicated by the absence of AM pulses on the audio carrier.
Each transmitted video signal is further identified with a digital tag code that is also amplitude modulated onto the audio carrier. The digital tag code identifies the video program content. Digital tag information is transmitted as follows: After the first horizontal sync pulse following vertical interval, and approximately one third way between two horizontal sync pulses, an AM pulse is provided as a logic one bit which represents the start bit of the digital tag code. Further data bits representing the digital tag information bits (each approximately one third way between consecutive horizontal sync pulses) are transmitted one bit per horizontal line. The presence of an AM pulse indicates a logic 1; the absence of an AM pulse indicates a logic 0.
A scrambled signal is usually detected by detecting the presence of valid digital tag data on a video signal. In the prior art, if tag data is detected and is unchanged for several consecutive fields, then the descrambler is enabled. The descrambler then restores the proper amplitude relationship between the video signal portion and the horizontal and vertical synchronizing pulses. However, there is often some amplitude modulation on the audio carrier due to adjacent channel energy, noise, or when changing channels. Such spurious audio carrier AM sometimes causes the descrambler to be enabled on clear signals, which interfers with normal signal reception.